Just to be sure I get this right: There is a new setting at the advanced question settings which defaults to the new behavior, so all newly created questions use the new behavior.
If I update from an older version though, this setting doesn't use the default behavior but is changed for the according updated questions so they use the original, old behavior. Is that correct?

Just to be sure I get this right: There is a new setting at the advanced question settings which defaults to the new behavior, so all newly created questions use the new behavior.
If I update from an older version though, this setting doesn't use the default behavior but is changed for the according updated questions so they use the original, old behavior. Is that correct?

Correct. Existing surveys will not have this attribute at all. When the attribute is missing, the old behavior is used (preventing users from skipping questions that have min_num_value or equals_num_value). If the author edits the question, that attribute is created, and defaults to the new behavior. So, authors editing such questions should pay attention to whether they want the old or new behavior.